1. Field of the Invention
This invention relates generally to heaters. More specifically, the present invention relates to a linear heater which can be advantageously used in an office automation apparatus such as a photocopier or electrophotographic printer for fixing images on a paper sheet for example.
2. Description of the Prior Art
Various types of linear heaters are known for fixing images (deposited toner) on a paper sheet in photocopiers or electrophotographic printers (e.g. laser beam printer). Typical examples include a lamp heater and a roller heater.
However, the lamp heater and roller heater are equally disadvantageous in that there is a limitation in reducing size (thickness) and cost. Further, the lamp heater is easily damaged due to the nature of material, whereas the roller heater has a complicated structure due to the necessity of incorporating plural heating elements within the roller.
To eliminate the problems of the conventional heaters, it has been proposed to use a strip heater for fixing images on a paper sheet in electrophotography. For the convenience of explanation, a typical arrangement of a prior art strip heater is shown in FIG. 14 of the accompanying drawings.
As shown in FIG. 14, the typical prior art strip heater comprises an elongate insulating substrate 100 having a surface formed with a printed resistor strip 101. Each end of the resistor strip 101 is connected to an enlarged terminal electrode 102 made of silver for connection to a power source (not shown). The resistor strip 101, which is made of e.g. silver-palladium alloy, generates heat when a current is passed therethrough.
Obviously, the prior art strip heater is very simple in arrangement. Further, the strip heater can be made very thin and light by reducing the thickness of the substrate 100. Moreover, the strip heater is also advantageous in that the time required for warming up is very short. However, the prior art strip heater still has the following problem.
Specifically, the enlarged terminal electrodes 102 made of silver are the portions where heat dissipation occurs most easily. Thus, when the resistor strip 101 is made to have a constant width over the entire length thereof, an uneven temperature distribution will result in which the surface temperature of the resistor strip is lower near the respective electrodes than at a central portion, as shown in FIG. 15. As a result, the heater will have an effective heating length A' which is non-negligibly smaller than the overall length A of the resistor strip 101. If the entire length A of the resistor strip 101 is utilized for heating, uneven image fixation will result. On the other hand, an attempt to increase the effective heating length A' will inevitably result in unacceptable elongation of the heater.
The above-described problem of the typical prior art strip heater can be solved by forming a resistor strip 101' which has a pair of end portions 101a' progressively reducing in width toward the respective electrodes 102, as shown in FIG. 16 and as taught in U.S. Pat. No. 5,068,517 to Tsuyuki et. al (Patented: Nov. 26, 1991; Filed: Aug. 22, 1989). However, such a solution gives rise to a new problem that the narrower end portions 101a' of the resistor strip 101' are more easily broken because, in spite of the reduced width, the narrower end portions 101a' generate a greater amount of heat than the central portion.